Pneumatic brake systems which use pressurized air to operate the service brakes and possibly also to release the parking or safety brakes are known in the art. Pneumatic systems are particularly desirable for tractor trailer combinations which are frequently connected/disconnected from one another. The pressurized air (so-called "supply pressure") is stored in a reservoir which is charged by a compressor through a series of check valves and/or including a pressure protection valve. In these pneumatic brake systems, the driver's brake pedal or other controls directs the flow of so-called "control air." The brake pedal, when applied, opens the reservoir air supply and sends control air, which generates control air pressure, to a relay valve. The relay valve, in response to the control air pressure, connects the reservoir air supply to the brake actuators, which applies mechanical force to the brakes.
In these truck systems, the pressure generated by the driver's foot on the pedal is not applied directly to the brakes; rather, it is used only to control the relay valve which in turn delivers air pressure from the reservoir to the brake actuators, applying mechanical force to the brakes. While the truck is in use, the supply air pressure remains at a high level. It may fluctuate somewhat, but it generally remains above 90 psig (pounds per square inch, gauge). When the mechanical parking brakes of the truck are needed and the pneumatic brakes are no longer needed, the brake system is normally de-pressurized and the trailer supply line air pressure drops to zero psig (or atmospheric pressure), thus applying the trailer parking brakes.
In general, pressurized air is an effective means to control the brakes; however, contaminants introduced either intentionally or accidentally can have detrimental effects on the system. For example, in cold weather, users often inject alcohol and other de-icing compounds into the air brake system to keep the brake components from freezing. Alcohol is detrimental to the valves because it removes necessary lubricants from them and may also degrade seals and hoses. Alcohol which builds up in the brake system may create a hydraulic-type system (which is a slower pressure transmitter than a dry pneumatic system) and/or vary the operating characteristics of the system due to the relative incompressibility of the liquid. Moreover, the alcohol or other contaminants build up at the relay valve because it is the last device in the control air system; this build-up hampers brake control.
Air dryers are commonly used in pneumatic brake systems to remove condensates and other contaminates prior to delivery of air to storage reservoirs and the like. But contaminates can be introduced down stream from the air dryer, thus they remain in the trailer brake system.
U.S. Pat. No. 5,154,204 to Hatzikazakis discloses a drain valve for removing moisture from these air dryers. The drain valve includes a spring which normally biases a ball against a seat thereby closing the valve. Periodically, pneumatic pressure is applied to a piston which unseats the ball and opens the valve, allowing any moisture accumulated in the air dryer to drain by the force of gravity. The '204 patent does not disclose pressurized ejection of moisture and other contaminants and the device taught is not generally usable to remove moisture which accumulates downstream in the braking system, e.g., in the relay valve.
U.S. Pat. No. 3,967,706 to King discloses a brake actuator housing having a passage through which pressurized air may pass to purge contaminants which may enter the housing due to the environment in which the brake is used. The device taught by King provides for purging contaminants from the supply side of the system; however, it does not provide for purging contaminants from within the control side of the system and thus is not effective in avoiding the above-mentioned problems.
What is desired therefore is a device which periodically purges contaminants, such as alcohol, from a pneumatic brake system and specifically from the control side of the system and/or from a relay valve in order to maintain optimal response and control of the pneumatically-controlled brake system. A device which provides for the pressurized ejection of contaminants is also desired.